As is known, extensible support legs schematically comprise a telescopic column which is fixed to the vehicle, and a contact organ which is fixed to the lower end of the telescopic column.
The contact organ is generally a platform or a small wheel.
The telescopic column comprises at least two coaxial tubes, of which one is an external tube and another is an internal tube which is slidably inserted in the external tube.
The contact organ is fixed to the lower end of the internal tube which projects from the external tube.
The lengthening and shortening of the telescopic column are obtained by a manoeuvring screw which is inserted in the coaxial tubes.
The manoeuvring screw is axially constrained to the external tube, and is screwed into a threaded volute axially constrained to the internal tube.
Thanks to this solution, the rotation of the manoeuvring screw causes an axial sliding of the internal tube in an upwards direction or in a downwards direction.
In the type of support leg the present invention particularly relates to, the rotation of the manoeuvring screw is activated by a drive shaft.
The drive shaft is connected to the manoeuvring screw by an intermediate gear and is rotated by a motor or a manually-activated handle.
When the vehicle is to be stabilised on the ground, the telescopic column is extended up to when the contact organ rests on the ground, after which it is further extended such as to raise and support at least part of the load.
In order to reduce the torque that is it necessary to apply in the second extending stage, with the aim for example of reducing the power of the motor or the operator's exertion if there is a manually-operated handle, or raising a given load with a given maximum torque applicable on the handle, an intermediate gear can be used for realising a rather high transmission ratio between the drive shaft and the manoeuvring screw.
The greater the transmission ratio of the intermediate gear, the greater the number of revolutions required on the drive shaft in order for the internal tube to slide in the external tube by a given quantity.
This fact can represent a drawback, as it means in particular that the first stage of extension of the telescopic column, i.e. the stage which is necessary for resting the contact organ on the ground, can become an extremely slow operation.
To obviate or at least reduce this drawback, a known solution is to interpose a mechanical gear change between the drive shaft and the manoeuvring screw, which can produce at least two different transmission ratios.
For example, patent application CA 2 659 294 illustrates a support leg with a mechanical gear change that comprises two gears with a different transmission ratio. Each of the gears comprises a first cog wheel mounted on the drive shaft, which enmeshes with a second cog wheel mounted on an intermediate shaft, and a third cog wheel mounted on the intermediate shaft, which in turn enmeshes with a fourth cog wheel mounted on the manoeuvring screw. The first cog wheel of each gear is rotatably idle on the drive shaft, which is axially mobile, in order to activate an enmeshing system which renders it rotatably solid with the first cog wheel of one or the other gear.
US patent 2008/0315570 describes a support leg having a mechanical gear change which also comprises an intermediate shaft, kinematically interposed between the drive shaft and the manoeuvring screw. The intermediate shaft is connected to the manoeuvring screw via a single gear, while it is connected to the drive shaft via two further gears having different transmission ratios. In this case too, the drive shaft is axially mobile in order to activate an enmeshing system which renders one or the other of the further gears active.
EP patent EP 1 350 701 also describes a mechanical gear change provided with an intermediate shaft, which is connected to the drive shaft by means of two gears with a different transmission ratio, selectively activatable by means of an enmeshing system, while it is connected to the manoeuvring screw by a single gear with a fixed transmission ratio.
The same constructive principle is also described in U.S. Pat. No. 5,238,266, which illustrates a support leg the gear change of which comprises an intermediate shaft which is constantly connected to the manoeuvring screw by means of a single gear. The drive shaft can be axially displaced between a position in which it is directly coupled to the intermediate shaft, or in a position in which it is connected to the intermediate shaft by means of a reducer group.
US patent application US 2001/020781, in the name of the same inventor as the previous patent, describes a constructional variant of the same solution.
Finally, German patent application DE 196 16 704 describes a support leg of traditional type, wherein the manoeuvring screw is connected to an intermediate shaft by means of a fixed gear, while the intermediate shaft is connected to the drive shaft by means of a gear change which is housed in a box located externally of the support leg.
All the known solutions have in common the fact of comprising a gear change which comprises an intermediate transmission shaft, kinematically interposed between the manoeuvring screw and the drive shaft.
Though providing good results, these solutions require the use and mounting of numerous mechanical components and cog wheels, making the gear change rather complicated to realise, as well as expensive.
Further, the gear changes realised in the above known solutions are very unwieldy, and increase the overall dimensions of the support leg, which obviously makes a larger space necessary for mounting thereof on-board vehicles.
An aim of the present invention is to make available a support leg with a gear change that is simpler, more compact and less expensive in relation to the known solutions, but which is equally functional and effective.
A further aim is to provide a gear change which is in any case able to effectively limit the torque to be applied to the drive shaft in the second extension stage of the support leg, during which the load is raised, guaranteeing however a sufficient rapidity of the first stage of extension during which the contact organ is neared to the ground.
A further aim of the invention is to reach the above-mentioned objective in the ambit of a simple, rational and relatively inexpensive solution.